The present invention relates generally to integrated circuits, and more specifically to electrical components of integrated circuits.
Analog integrated circuits (ICs) are now being extensively used, for example, in wireless radio frequency (RF) applications such as cellular telephones where high frequencies are encountered. Many analog ICs include inductive elements, such as inductors, formed by a conductor. Inductive elements with a relatively high quality (Q) factor, or low loss, are preferably used in analog ICs. As a result, the analog integrated circuits have superior performance, including selectivity, noise figure, and efficiency. Relatively high Q inductors have been formed on insulating bulk semiconductors, such as gallium arsenide.
Most integrated circuits, however, are formed on silicon. In comparison to gallium arsenide ICs, silicon ICs can be fabricated relatively inexpensively. Also, analog and digital circuits may be readily combined on silicon ICs. However, unlike gallium arsenide, silicon is a conductive bulk semiconductor. As a result, conventional inductive elements formed on silicon are relatively lossy, and thus have relatively low Q factors. For example, Q factors of 3 to 8 are reported for inductors fabricated on silicon in Nguyen et al., xe2x80x9cSi IC-compatible inductors and LC Passive Filters,xe2x80x9d IEEE Journal of Solid-State Circuits, vol. 25, no. 4, p. 1028-1031, 1990, herein incorporated by reference.
An inductor formed on an IC 101 may be a conventional rectangular spiral inductor 103, as illustrated in FIG. 1A. The conventional rectangular spiral inductor 103 includes substantially parallel conductive branches 121 that are mutually coupled to increase the rectangular spiral inductor""s 103 effective inductance.
The conventional rectangular spiral inductor 103 is formed in the following manner. A first conductor 109 is patterned on the IC 101. Then, an insulator, such as resist, defining the location of air bridges 105, is patterned on the IC 101. Next, a second conductor 107 is patterned on the IC 101. However, where an air bridge 105 is to be formed, the insulator separates the first and second conductors 107, 109. Finally, conventional air bridges 105 are formed by removing the insulator.
Conventional air bridges 105, in this example, permit the two conductors 107, 109 to cross one another, without making electrical contact, as illustrated in FIG. 1B. Conventional air bridges 105 are formed by substantially perpendicular conductors 107, 109 to diminish undesired magnetic coupling between the conductors 107, 109. Further, relatively low-dielectric-constant air typically separates the conductors 107, 109 to diminish undesired capacitive coupling between the conductors 107, 109.
FIG. 1C illustrates a prior art first order lumped element electrical model of the rectangular spiral inductor 103 that describes the electrical characteristics of the rectangular spiral inductor 103 below its self-resonant frequency. The self resonant frequency is the maximum frequency at which the rectangular spiral inductor 103 acts as an inductor. Above the self resonant frequency, for example, the rectangular spiral inductor may exhibit capacitive characteristics.
L is the effective inductance of the rectangular spiral inductor 103. The effective inductance represents the sum of both self and mutual inductances of the branches 121. The inductance, L, of the rectangular spiral inductor 103 is determined by (1) the length of the branches 121, (2) the spacing between the branches 121, and (3) the number of branches 121, or turns.
The other model elements are parasitics that result from the physical implementation of the rectangular spiral inductor 103. RDC and RSKIN EFFECT are respectively the lumped element equivalent DC and skin effect resistances of the conductors 107, 109. RDC is determined by the cross-sectional area, length and resistivity of the conductors 107, 109. RSKIN EFFECT is determined by the thickness of the conductors 107, 109. CS is a lumped element equivalent capacitance representing the interwinding capacitances between the parallel conductive branches 121. CS is determined by both the distance between adjacent branches 121, and the dielectric constant of the material proximate to those adjacent branches 121. The CpS are lumped element equivalent capacitances representing capacitances between the conductors 107, 109 and a ground plane under the IC 101 on which the rectangular spiral inductor 103 is formed. The CpS correspond to the width of the conductors 107, 109, and the thickness and dielectric constant of the material between the conductors 107, 109 and the ground plane. RSUBSTRATE is a lumped element equivalent resistance corresponding to substrate losses. The Q factor and self-resonant frequency of the rectangular spiral inductor 103 are a function of the reactances and resistances described by the electrical model of FIG. 1C.
To increase its Q factor, resistances and/or capacitances of the rectangular spiral inductor 103 should be reduced. One technique for reducing the Q factor of the inductor is disclosed in J. N. Burghartz et al., xe2x80x9cIntegrated RF and Microwave Components in BiCMOS Technology,xe2x80x9d IEEE Trans. Electron Devices, vol. 43, no. 9, pp. 1559-1570, 1996 (herein after the xe2x80x9cBurghartz Articlexe2x80x9d), herein incorporated by reference. The Burghartz Article discloses inductors, on silicon ICs, whose conductors are displaced above the silicon, and are encased in oxide. These inductors have Q factors exceeding 10. The higher Q factors arise, in part, because the inductors, disclosed in the Burghartz Article, have relatively lower values of Cp because the conductors are farther displaced from the IC ground plane by the oxide.
Further, the inductors disclosed in the Burghartz Article require a complex five-level metal silicon technology that is more complicated than conventional two- to four-level metal silicon technologies. Therefore, there is a need for inductors having relatively high Q factors that can be formed with conventional silicon technologies.
The present invention provides a method of forming air bridges, on a substrate or an integrated circuit, which may be used to form inductors and other devices. A first insulator is formed on a base layer. A first conductor is formed on the first insulator. The first conductor is patterned. A second insulator is formed over the first insulator. A via hole is formed in the second insulator. A second conductor is formed on the second insulator, and is electrically coupled to the first conductor by the via hole. The second conductor is patterned. A cavity is formed under the second conductor, and in the first and second insulators. In one embodiment, the first and second conductors form air bridges.
In another embodiment, a support structure is formed during the step of forming the cavity. In yet another embodiment, a conductive pad is formed over the support structure during the step of patterning the second conductor.
In a further embodiment, the present invention provides an air bridge or inductive element on a substrate or integrated circuit. A first insulator is formed on a base layer. A first conductor is formed and patterned on the first insulator. A second insulator is formed on the first insulator. A via hole is formed in the second insulator. A masking layer is developed on the integrated circuit. A cavity, defined by the developed masking layer, is formed in the first and second insulators. The cavity is filled with a polymer. The integrated circuit is cleaned. A second conductor is formed on the polymer, and coupled to the first conductor by the via hole. The second conductor is patterned. In yet a further embodiment, the cavity is filled with a polymer that is foam.
In yet a further embodiment, the inductive element includes a second via hole in the support structure that couples the first and second conductors. In another embodiment, the cavity is filled with a polymer. In yet a further embodiment, the the polymer is a foam.
In another embodiment, an inductive element on a substrate, or an integrated circuit, comprises a base layer. A first conductor is buried in the base layer. An insulator is formed on the base layer. A second conductor, having first and second branches that are substantially parallel, is formed on the second insulator. A plug, formed in the base layer, is coupled to the first conductor. A via hole, formed in the insulator, couples the plug to the second conductor. A cavity, under second conductor, is formed in the insulator. A support structure, in the cavity, props up the second conductor above the base layer.
In yet a further embodiment, an inductive element is formed, on a substrate or an integrated circuit, with a low dielectric inorganic insulator. A first insulator is formed on a base layer. A first conductor is formed on the first insulator. The first conductor is patterned. A second insulator is formed, over the first insulator, from the low dielectric inorganic insulator. A portion of the second insulator is oxidized. The oxidized portion of the second insulator is removed. A via hole is formed in the second insulator. A second conductor, formed on the second insulator, is coupled to the first conductor by the via hole. The second conductor is patterned.
It is a benefit of the present invention that the inductive elements described above have an enhanced Q factor. It is a further advantage of the present invention that the inductive elements described above have an enhanced self-resonant frequency. It is yet a further benefit of the present invention that the inductive elements described above can be formed in conjunction with standard silicon IC processes.
The inductive elements described above can be incorporated in a receiver and/or a transmitter of a communications systems. As a result, the communications system will exhibit higher selectivity and efficiency, and lower noise figure.